The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing and completing wells in which acid gases reside, are injected, stored or recovered.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone.
The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks. However, over time the cement sheath can deteriorate and become permeable. Alternatively, the bonding between the cement sheath and the tubular body or borehole may become compromised. The principal causes of deterioration and debonding include physical stresses associated with tectonic movements, temperature changes, pressure changes inside the casing and chemical deterioration of the cement.
There have been several proposals to deal with the problems of cement-sheath deterioration. One approach is to design the cement sheath to mechanically survive physical stresses that may be encountered during its lifetime. Another approach is to employ additives that improve the physical properties of the set cement. Amorphous metal fibers may be added to improve the strength and impact resistance. Flexible materials (rubber or polymers) may be added to confer a degree of flexibility to the cement sheath. Or, cement compositions may be formulated to be less sensitive to temperature fluctuations during the setting process.
A number of proposals have been made concerning “self-healing” concretes in the construction industry. The concept involves the release of chemicals inside the set-concrete matrix. The release is triggered by matrix disruption arising from mechanical or chemical stresses. The chemicals are designed to restore and maintain the concrete-matrix integrity. None of these concepts are immediately applicable to well-cementing operations because of the need for the cement slurry to be pumpable during placement, and because of the temperature and pressure conditions associated with subterranean wells.
More recently, self-healing cement systems have been developed that are tailored to the mixing, pumping and curing conditions associated with cementing subterranean wells. One approach is to add superabsorbent polymers that may be encapsulated. If the permeability of the cement matrix rises, or the bonding between the cement sheath and the tubular body or borehole wall is disrupted, the superabsorbent polymer becomes exposed to formation fluids. Most formation fluids contain some water, and the polymer swells upon water contact. The swelling fills voids in the cement sheath, restoring the low cement-matrix permeability. Likewise, should the cement/tubular body or cement/borehole wall bonds become disrupted, the polymer will swell and restore isolation. Another approach involves the addition of rubber particles that swell when exposed to hydrocarbons. Like the superabsorbent polymers, the swelling of the rubber particles restores and maintains zonal isolation.
Some oil and gas fields have formations whose fluids contain acid gases such as carbon dioxide and hydrogen sulfide. Such wells may be challenging from a zonal isolation point of view. Carbon dioxide injection is a well-known enhanced oil recovery (EOR) technique. In addition, there are some oil and gas wells whose reservoirs naturally contain carbon dioxide.
A relatively new category of wells involving carbon dioxide is associated with carbon-sequestration projects. Carbon sequestration is a geo-engineering technique for the long-term storage of carbon dioxide or other forms of carbon, for various purposes such as the mitigation of climate change. Carbon dioxide may be captured as a pure byproduct in processes related to petroleum refining or from the flue gases from power plants that employ fossil fuels. The gas is then usually injected into subsurface saline aquifers or depleted oil and gas reservoirs. One of the challenges is to trap the carbon dioxide and prevent leakage back to the surface; maintaining a competent and impermeable cement sheath is a critical requirement.
Oil and gas that contains elevated amounts of hydrogen sulfide are called “sour.” It has been estimated that 15 to 25% of natural gas in the United States may contain hydrogen sulfide. Worldwide, the percentage could be as high as 30%. Hydrogen sulfide is a toxic substance; therefore, it is important to prevent it from escaping through the cement sheath into aquifers or to the surface. Furthermore, hydrogen sulfide is corrosive to steel, and maintaining a competent cement sheath is essential to prevent casing deterioration during the life of the well.